Polyester resins are widely used as molding products or synthetic fibers. Increasing the strength of a molding product or fiber may be probable by increasing the degree of polymerization of the polymer. However, when the degree of polymerization of the polymer is increased, the melt viscosity thereof is increased as a matter of course impairing the workability thereof and reducing the productivity. Although elevation of the melting temperature is one approach for reducing the melt viscosity, the decomposition of the resin is accelerated to reduce the degree of polymerization of the polymer. Thus, it cannot be achieved to produce a molding product or a fiber having a high degree of polymerization and high strength.
Polyester resins are widely used on account of their various excellent characteristics. However, they have low dyeability and can hardly be dyed except with disperse dyes. Among various proposals to improve the dyeability of polyester resins, copolymerization of an isophthalic acid component containing a sulfonic acid salt moiety enables the polyester to be dyed with cationic dyes, as disclosed e.g. in JP-B-34-10497 (1959) (the term "JP-B" as used herein means an "examined published Japanese patent application").
However, such a polymerization system causes a considerably increased melt viscosity of resin due to the thickening effect of the sulfonate-containing isophthalic acid component, and the resin thus obtained has a low moldability. Therefore, it is difficult to produce a cationic dye-dyeable polyester resin having a high degree of polymerization and high strength.
In order to overcome these problems, the addition of a lubricant has been suggested. For example, the addition of ethylenebisstearamide, stearic acid and stearyl alcohol as a lubricant to a resin compound decreases the melt viscosity, nevertheless the degree of polymerization of the resin also decreases.
On the other hand, aromatic ether compounds, alkyl diphenyl compounds and aromatic imide compounds have been known as a melt viscosity depressant for polyester resin, as disclosed in JP-A-3-223382 (1991) and JP-A-3-223383 (1991) (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, as the compatibility of these compounds with polyester resin is not sufficient, a prolonged mixing period is necessary in order to obtain an effective depression of melt viscosity. Such a prolonged mixing period is not favourable for obtaining a polyester resin compound with a retained high degree of polymerization, because the degree of polymerization is considerably affected by the time and temperature during the mixing.